Patients with sickle cell disease (SCD) experience recurrent episodes of vaso-occlusive crises(VOC). Following a VOC, patients often develop pulmonary symptoms resulting in a potentially life-threatening condition termed acute chest syndrome (ACS). ACS is the second most common cause of mortality in SCD. In the National Acute Chest Study, 38% of ACS cases in SCD were attributed to infection, fat embolism and infarction, while no identifiable cause was found in the remaining 62% of ACS cases. Recruitment of monocytes (Mo)/polymorphonuclear neutrophils(PMN) into the alveolar compartment is an important feature of acute lung injury. Leukocytosis, in the absence of infection, is commonly seen in SCD patients and is a predictor of severity and ACS. Studies in transgenic sickle mice (Tg HbS) reveal increased accumulation of leukocytes in the lung in response to an experimental lung insult. We find that endothelin-1(ET-1) is released from cultured human pulmonary endothelial cells (HPEC) in response to deoxygenated sickle(SS) RBCs and initiates leukocyte transmigration.The goal of this project is to test the hypothesis that the interaction of SS RBC with HPEC initiates cellular signaling to cause Mo/PMN to transmigrate from the lumen of the blood vessel into the alveolar compartment, wherein activated Mo/PMN cause injury to the pulmonary alveolar epithelial cells (PAEC). SpecificAim 1, examines the effectof SS RBC with cultured HPEC on the transmigration of Mo/PMN and the role of endothelin-1 (ET-1) in this process. Specific Aim 2 delineates the effect of conditioned medium, elaborated from the interactionof SS RBC with HPEC, on the migration of Mo/PMN from the basolateral to apical direction across AEC monolayers. Specific Aim 3 explores the mechanism of transmigration of Mo/PMN in a co-culture model of HPEC and PAEC. Finally, Specific Aim 4 employs a Tg HbS mouse model to induce ACS and examines the effect of ET-1 receptor antagonists in preventing the accumulation and activation of Mo/PMN and resultant lung injury. Together, these coordinated investigations will improve our understanding of the molecular mechanisms by which Mo/PMN accumulatein the alveolar compartment in SCD leading to ACS and provide a rationale for therapeutic approaches to ameliorate ACS.